In rifles and other long guns the primary structure of the receiver, action, barrel and associated components is usually mounted in a wooden or plastic stock and secured by screws through the stock into one or more spaced apart positions on the metal structure. The assembly is thus supported only at the points of attachment unless the stock is built up or modified to bed the structure closely into the stock. This is usually accomplished by use of resin materials and glass fiber or other reinforcement, with the rifle structure seated in the still wet or pliable material to ensure a close fit, or by careful shaping and machining to fit the parts.
In a gas operated semi-automatic rifle, a portion of the propellant gas is diverted from the barrel after passage of the bullet to drive a slide mechanism rearward, which actuates the bolt to extract the empty casing and load a new round. The slide mechanism is mounted under the barrel and the stock is-cut away to provide operating clearance. This makes it impractical to support the barrel between the receiver and the gas diverting block, the latter usually being one of the mounting points to the stock. With the structure supported only at small specific points, the firing and recoil loads cause transient disturbances that are adverse to consistent barrel motion, contributing to inaccuracy of the aim. This is especially true of lightweight sportier type barrels where complex harmonic motions are induced from gas pulse reactions of the actuator, as well as dynamic interference from inertia block movements. Further, the cantilevered forward barrel section can develop a whipping action which will loosen the supports and cause the rifle to lose its precise sighting alignment.
In normal production of rifles it is not feasible to maintain the precise tolerances necessary for individually bedding the metal assembly. Also, with wood and plastic stocks which are affected by temperature, humidity and other factors, such close tolerances would not be stable. If a secure mounting is essential, as for precision target shooting, custom fitting becomes necessary and this can be time consuming and expensive.